I agree with Jeff's opinion - for more ubiquitous watches, I think it's a waste of time and as a seller, the price you want is the price you should list it for.

For rare examples, however, I think it's the best format to use - mainly because it gives everyone who would want a shot at buying the watch a chance to own it, not just the first person that happened to see the ad and respond. I've sold a 60's UG Tri-Compax and Collector Grade 105.012-66CB Speedy this way, and both times it achieved a) a fantastic (above market) result for me, and b) resulted in passionate buyers ending up with the watch. Both were profusely thanking me upon the word that they had "won" the auction. I think it's a cool dynamic for both buyer and seller.

Just my two cents...